Lake Stymfalia is known from ancient times. According to legend, the sixth Labor of Hercules was to kill the ‘Stymfalides Ornithes’, birds with sharp beaks and bronze claws that were nesting in the lake, destroying the crops of farmers, eating their livestock and terrorizing the local residents. Hercules startled the birds with the sound of brass rattles built by the god Hephaestus, and given to him by the goddess Athena. When the startled birds took off flying, Hercules killed them with arrows dipped in the poisonous blood of the mythical monster, the Lernaia Hydra. The lake sits at an altitude of 680m. The waters of the nearby mountains (Zireia and Oligyrtos) empty into it, and that is why the level of the water fluctuates. Stymfalia lake is one of the most important wetlands of the northeastern Peloponnese. Its waters have been used for irrigation, and flow through the ancient plumbing of Hadrian's aqueduct, built in the 2nd century AD.